1. Field of the Invention
The present invention relates generally to improvements in apparatus for the non-tubular cracking of heavy hydrocarbon feeds for the purposes of producing ethylene and other valuable olefin products as well as chemical grade synthesis gas.
2. Prior Art
A great deal of prior work has been done in connection with the cracking of hydrocarbon feed stocks to obtain basic chemicals such as ethylene, acetylene and propylene. Presently, the most common approach involves the cracking of a hydrocarbon feed in the presence of steam in a fired tubular furnace. In such a steam cracking process the thermal energy of the combustion gases is transferred to the feed through the metal walls of the tubes and therefore the tube metallurgy becomes one of the limiting factors with respect to the maximum cracking temperature which is usually several hundred degrees below that which can be achieved in a non-tubular cracking process such as the diacritic cracking process described herein. To obtain proper cracking conditions, the residence time in a steam cracker must be substantially longer to compensate for the lower temperatures. For example, residence time of about 0.25 to 0.50 seconds is typical in many modern steam cracker designs. Such longer residence times lead to a furnace effluent hydrocarbon composition that is significantly different from that obtained from a process utilizing a non-tubular reactor. For example, acetylene and PG,4 ethylene yields are generally lower but the yields of propylene, C.sub.4 olefins and pyrolysis gasoline are usually somewhat higher.
In view of the shortcomings of steam cracking considerable work has been done in connection with the thermal cracking of hydrocarbons to obtain higher yields of ethylene or in some instances acetylene. For example, U.S. Pat. No. 3,790,838 deals with the general process of a single pass cracking method for the production of mixtures of acetylene and ethylene (with the primary emphasis on acetylene) in which a hydrocarbon feed stock is cracked by "thermal shock" when contacted with hot gases produced by the combustion of a fuel. The apparatus used comprises a combustion furnace to which may be supplied steam, oxygen and a fuel, with the hot combustion gases passing into a reactor having a venturi constriction positioned near the combustion furnace exhaust port and the feed stock inlet. Quenching at the reactor outlet is provided by coolant spray nozzles. No provision is made in the apparatus for the minimization of coking problems which typically result in such a process, especially when heavy hydrocarbon feed stocks are used as are contemplated for the preferred embodiment of the present invention.
U.S. Pat. No. 2,751,334 discloses a continuous flash coking process to vaporize hydrocarbons and produce coke from a hydrocarbon feed. The apparatus receives a retort gas and directs the gas tangentially into a generally circular reaction chamber as a result of baffles therein. The feed particles are sprayed into the vortex of hot gases with the products of the process ultimately proceeding axially through a lock hopper to a coke draw-off line.
U.S. Pat. No. 3,178,488 discloses apparatus for the production of unsaturates by the non-uniform mixing of paraffin hydrocarbons with hot combustion gases. The apparatus comprises a combustor for providing hot combustion gases to which the hydrocarbon feed is controllably injected into at the throat of a venturi. As a result of the controlled injection, two cracking zones are formed within the reaction zone, the interior zone being characterized by a high velocity gas, high temperature and thorough mixing by the combustion gases with the hydrocarbon cracking feed, and the outer cracking zone being characterized by lower velocity, lower temperature and less than thorough mixing of the feed stock and combustion gases. The apparatus includes a water spray quench at the reactor outlet.
U.S. Pat. No. 2,823,243 discloses a process and apparatus for the pyrolysis of hydrocarbons wherein hydrogen and oxygen are burned and mixed with a gaseous hydrocarbon stream. Means are provided to establish a helically moving blanket of tempering gas around the combustion gas to cool the combustion gas. A venturi is provided downstream of the region of injection of the feed stock, with a second venturi being placed at right angles thereto subsequent to the quench. Similarly U.S. Pat. No. 2,767,233 discloses apparatus for use in the thermal transformation of hydrocarbons utilizing a venturi to provide very high velocity through the reactor, though few structural details of the apparatus are specifically set forth. U.S. Pat. No. 3,408,417 discloses apparatus for the thermal cracking of hydrocarbons utilizing a throat or venturi between the combustor and the reactor to provide sonic or supersonic velocities in the reactor. The feed stock is injected just upstream of the throat. Other apparatus also using a constriction to produce high velocity is shown in U.S. Pat. Nos. 2,905,731 and 2,941,021.
Many other prior U.S. patents show various specific configurations for pyrolyzing apparatus or portions thereof. By way of example, U.S. Pat. No. 2,822,411 discloses a specific reactor and feed stock injection configuration with U.S. Pat. No. 2,934,410 discloses in detail a two-stage burner apparatus. U.S. Pat. No. 2,498,444 discloses an apparatus for the production of acetylene, that apparatus including a provision for the helical flow of air around the upstream or main combustion portion of the reaction tube prior to its injection into the tube. U.S. Pat. No. 2,985,698 discloses apparatus for pyrolyzing hydrocarbons using a two-stage pyrolysis, the apparatus including either a ceramic lined or fluid cooled reactor chamber.
U.S. Pat. No. 3,073,875 discloses an apparatus for the preparation of acetylene which includes means for providing a helical curtain of fluid on the inside walls of the pyrolysis chamber so that the reaction zone is surrounded and confined by the fluid screen flowing helically downward, thereby preventing the build-up of coke in that region. U.S. Pat. No. 3,843,744 discloses apparatus for controlling coke in the pyrolysis of hydrocarbons through acetylene and hydrogen which apparatus includes means for mixing stream and/or an inert gas at at least one critically located point in the system downstream from the feed injection. In addition to these patents, the following patents disclose various specific details of other equipment for the manufacture of hydrocarbon products by cracking and oxidation processes: U.S. Pat. Nos. 2,188,982; 2,630,461; 2,644,744; 2,813,138; 2,870,231; 2,882,960; 3,006,944; 3,285,847; 3,498,753; and 3,542,894.
U.S. Pat. No. 4,035,137 discloses a burner unit which includes means for introducing a primary stream of decoking air along the interior surface of the cylindrical wall of the burner. A second stream of decoking air may also be injected at a later section of the burner.
In addition to the above-mentioned patents, the Applicants are also aware of certain experimental apparatus and work which has been conducted in connection with a process for the thermal cracking of a hydrocarbon (i.e. hexane) and do ethylene, acetylene and other bi-products such as propylene and butadiene. Such a process was of an experimental nature to determine yields and feasibility and therefore involved only a short term testing which did not consider the prevention of coking which is a severe practical problem in commercial installations. There were no "decoking" apparatus or techniques used in the combustor or reactor sections of the experimental apparatus. Also the experimental process used hexane as a feed stock and accordingly the process and related equipment were not designed to handle and inject heavier and more troublesome hydrocarbon feed stocks such as are used with the present invention apparatus. The reactor used on this program had a water cooled combustor section terminating at a throat after which the flow area took an essentially step increase to the feed stock injection region. Thereafter the flow area flared outward to a larger cylindrical reactor terminating again in a sharply defined venturi for exhausting the products into a quench region for capture and analysis. The reactor was neither insulated nor water cooled though as previously mentioned, the apparatus was neither intended or suitable for any use other than a highly intermittent experimental use.